Light Gate Circuit

Discussion in 'The Projects Forum' started by bambicomp, Sep 23, 2008.

  1. bambicomp

    Thread Starter Member

    Sep 23, 2008
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    Hello I am trying to build a light gate system. The idea being that the beam is broken when you pass the first point and a counter is started until the second beam is broken. I have seen other posts on her on about IR emitter and photo detector. I have tried this. However they pick up ambient light and also I need a spacing of bout a meter at least between the emitter and detector. Can anyone suggest a suitable emitter / detector pairing or even another way around it?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Consider using an inexpensive low-power red laser (like a laser pointer) and a photodiode or phototransistor. You can pick up laser pointers many places for just a few dollars. The collimated (focused) beam is quite narrow and bright. You could put the photodiode/phototransistor inside a small-diameter tube, which would eliminate most of the ambient light.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    I like the laser approach as well. Especially with baffles or other means of restricting the receiver's field of view so that it doesn't "see" so much of the ambient light.

    You might also want to try modulating the LED or laser with a carrier, then at the receiver, demodulate and detect the carrier -this is pretty much the way IR remote controls operate, though they also use a bandpass filter. The addition of some IR filters also helps reject ambient light.

    The implementation might be improved by an automatic bias circuit as shown on sheet 7 of this patent:

    http://www.cappels.org/dcpat/7196690_B2.pdf

    Figure 6 is a block diagram of the detector without bandpass filtering.
     
  4. bambicomp

    Thread Starter Member

    Sep 23, 2008
    15
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    Thanks to all for your laser suggestions. I have done some reading up on them and think they will do the business. I have ordered some.

    Meanwhile while Im waiting for them, there is a second part to the project. I have to basically count the amount of time between when the first light is broken until the second light.

    What is the easiest way to do this. I have semi read about flip flops but not sure and how would I actually count the time?

    ANy diagrams or info would be much appreciated.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    First, you need to decide what kind of resolution is required; eg: 1/1000 of a second, 1 second, 1 minute, 1 hour, 1 day, etc.

    Second, you need to decide what range of these time periods you need to be able to measure and display.

    Attempting to maintain a high resolution over a long period of time can be done, but it will require a very complicated circuit if constructed from discrete components.
     
  6. bambicomp

    Thread Starter Member

    Sep 23, 2008
    15
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    Hello not sure what you meant by resolution.

    I'll try my best to explain what I want and hopefully that might answer your question. The two beams would be approximately 10 - 20 meters apart. SO would take maximum 30 seconds to break the two of them (thinking someone with poor mobility being worst case.) So the acuracy of the timing would need to be something reasonable for this. Does this answer give you an indcation?
     
  7. DickCappels

    Moderator

    Aug 21, 2008
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    So...what "accuracy" are you shooting for. In general, the higher the accuracy, the higher the cost.

    BTW, for future reference: I have found that laser pointers are a good source of cheap lasers (imagine...cheap lasers!)
     
  8. bambicomp

    Thread Starter Member

    Sep 23, 2008
    15
    0
    Hi I have got the lasers going perfectly. When beam is split i get 5 volts very clear and 0v when beam is in tact. Regarding the accuracy needed im looking for something reasonably accurate but not going to cost the world. I was rwading about flip flops as previously mentioned, is this what I would use?

    Im looking to time the first beam broken until the second beam is broken. SO can anyone send me a diagram or details of what I need to do
     
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